This invention relates to methods and apparatuses for packaging microelectronic substrates.
Packaged microelectronic devices, such as memory chips and microprocessor chips, typically include a microelectronic substrate die encased in an epoxy protective covering. The die includes functional features, such as memory cells, processor circuits, and interconnecting circuitry. The die also typically includes bond pads electrically coupled to the functional features. The bond pads are coupled to pins or other types of terminals that extend outside the protective covering for connecting to buses, circuits and/or other microelectronic devices.
In one conventional arrangement shown in FIG. 1, a mold or cull tool 40 simultaneously encases a plurality of microelectronic substrates 30. The cull tool 40 can include an upper plate 42 removably positioned on a lower plate 41 to define a plurality of substrate chambers 45, an upright pellet cylinder 60, and a plurality of channels 46 connecting the substrate chambers 45 to the cylinder 60. A narrow gate 44 is positioned between each channel 46 and a corresponding substrate chamber 45. A cylindrical pellet 20 formed from an epoxy mold compound is positioned in the cylinder 60, and a plunger 50 moves downwardly within the cylinder 60 to transfer heat and exert pressure against the pellet 20. The heat and pressure from the plunger liquifies the mold compound of the pellet 20. The liquified mold compound flows through the channels 46 and into the substrate chambers 45 to surround the microelectronic substrates 30 and drive out air within the cull tool 40 through vents 43.
The mold compound in the substrate chambers 45 forms a protective covering around each microelectronic substrate 30. The residual mold compound in the channels 46 and in lower portion of the cylinder 60 forms xe2x80x9ccull.xe2x80x9d The cull has thin break points corresponding to the location of each gate 44. After the upper plate 42 is separated from the lower plate 41, the encapsulated microelectronic substrates 30 and the cull are removed from the tool 40 as a unit. The encapsulated microelectronic substrates 30 are then separated from the cull at the break points.
The mold compound that forms the pellet 20 is typically a high temperature, humidity-resistant, thermoset epoxy. One drawback with this compound is that it can be brittle and accordingly the corners of the pellet 20 can chip. One approach to addressing this drawback is to provide a shallow chamfer at the corners 21, as shown in FIG. 1. Another drawback with this compound is that it must be elevated to a relatively high temperature before it will flow through the cull tool 40. Accordingly, the cull tool 40 and the plunger 50 can be heated to improve the heat transfer to the pellet 20. Furthermore, the lower plate 41 of the cull tool 40 can include one or more protrusions 47 that can improve the flow of the mold compound within the cull tool 40.
Still another drawback with the molding process discussed above is that the cull cannot be easily recycled because it is formed from a thermoset material that does not xe2x80x9cre-liquifyxe2x80x9d upon re-heating. Accordingly, the cull is waste material that must be discarded, which increases the materials cost of producing the packaged microelectronic devices. One approach to address this drawback is to reduce the volume of the pellet 20 and, correspondingly, the channels 46 that define the shape and volume of the cull. For example, one conventional approach includes reducing the length and/or the diameter of the pellet 20. However, such pellets are not compatible with existing handling machines. For example, if the pellet length is decreased substantially, the length and diameter of the pellet will be approximately equal. The sorting and handling machines (not shown) that orient the pellets 20 for axial insertion into the cylinder 60 cannot properly orient the shorter pellets because the machines cannot distinguish between the length and diameter of the pellet. Furthermore, the handling machines are typically calibrated to reject undersized pellets on the basis of pellet length and accordingly would likely reject all or none of the reduced-length pellets.
The present invention is directed toward methods and apparatuses for packaging microelectronic substrates. A method in accordance with one aspect of the invention includes forming a pellet of uncured thermoset mold compound to have a first end surface, a second end surface opposite the first end surface, and an intermediate surface between the first and second end surfaces. The method further includes forming at least one cavity in the pellet and at least partially enclosing the microelectronic substrates by pressurizing the pellet and flowing the pellet around the microelectronic substrate.
A method in accordance with another aspect of the invention includes forming a pellet suitable for use with a pellet-handling apparatus configured to handle cylindrical pellets having a selected length, a selected radius less than the selected length, and a selected volume approximately equal to pi times the selected length times the square of the selected radius. The method includes forming a pellet material into a pellet body having a first end surface, a second end surface opposite the first end surface, and an intermediate surface between the end surfaces. The pellet body has a maximum length approximately equal to the selected length, a maximum cross-sectional dimension approximately equal to twice the selected radius, and a volume less than the selected volume by at least about 5%.
The invention is also directed to a pellet for packaging at least one microelectronic substrate. The pellet can include a pellet body formed from an uncured thermoset mold material. The pellet body has a first end surface, a second end surface facing opposite the first end surface, and an intermediate surface between the first and second end surfaces. The first end surface, the second end surface and the intermediate surface define an internal volume, and at least one of the surfaces and/or the internal volume has at least one cavity. In one aspect of this invention, the cavity has a generally spherical shape. In another aspect of this invention, the cavity can include a slot in the first end surface arranged transverse to the side surface. In still another aspect of this invention, the pellet body can have a generally right-cylindrical shape with a chamfered corner forming angles approximately 45 degrees between the first end surface and the side surface.
The invention is also directed to an apparatus for packaging a microelectronic substrate. The apparatus can include a mold body having a chamber with a first portion configured to extend at least partially around the microelectronic substrate and a second portion coupled to the first portion. A plunger is positioned in the second portion of the chamber and is moveable within the second portion of the chamber in an axial direction. The plunger has a side wall aligned with the axial direction and an end wall transverse to the axial direction. At least a portion of the end wall extends axially away from the side wall. In one aspect of this embodiment, the plunger is configured for use with a pellet having a cylindrical side surface and two end surfaces. Each end surface can have a cavity defining a cavity shape, and the end wall of the plunger can be shaped to be received in the cavity of the pellet.